Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!apple!mips!sgi!vjs@rhyolite.wpd.sgi.com
From: vjs@rhyolite.wpd.sgi.com (Vernon Schryver)
Newsgroups: comp.protocols.misc
Subject: Re: Realtime Protocols
Summary: token ring bounds are useless
Message-ID: <70903@sgi.sgi.com>
Date: 2 Oct 90 17:59:57 GMT
References: <24594@uflorida.cis.ufl.EDU> <32683@sparkyfs.istc.sri.com>
Sender: guest@sgi.sgi.com
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Organization: Silicon Graphics, Inc., Mountain View, CA
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In article <32683@sparkyfs.istc.sri.com>, levin@sparkyfs.istc.sri.com (Larry Levin) writes:
> 
> There are alternatives to TDMA.  A timed token protocol such as that
> used with 802.4 or FDDI will put a bound on response time and provide
> prioritization as well....


I don't know about 802.4, but I do know about FDDI.

It is true that the FDDI token ring does put a lower bound on the speed
with which the token rotates.  However, no one using the stuff cares.

First, the bound is valid only when the ring is working.  Any of a zillion
things can break the FDDI ring.   For example, if one station "sticks in
BEACON" and it does not do the RMT state machinery stuff, the entire ring
will be dead forever.  Since any network can be trashed by sufficiently
sneaky hardware or software failures, this complaint is not interesting.

Second, the latency between opportunities to transmit on a saturated FDDI
ring with the maximal number of stations and default parameters is a small
matter of hundreds of seconds.  You can improve the latency on a big,
saturated ring to about 6 seconds by reducing its bandwidth to arbitrarily
close to zero by decreasing TRT toward D_Max.  Of course, reducing the
bandwidth does wonders for increasing the likelihood of saturation.


The bound exists, but is useless to an honest person.



Vernon Schryver,    vjs@sgi.com